APOLAR MOLECULES: CHARACTERISTICS, IDENTIFICATION AND EXAMPLES - CHEMISTRY - 2025

The apolar molecules are those that present in their structure a symmetric distribution of their electrons. This is possible if the electronegativity difference of their atoms is small, or if the electronegative atoms or groups vectorly cancel their effects on the molecule.

Not always "apolarity" is absolute. For this reason, molecules with low polarity are sometimes considered nonpolar; that is, it has a dipole moment µ close to 0. Here we enter the field of the relative: how low must µ be for a molecule or compound to be considered non-polar?

Nonpolar molecule of BF3. Source: Benjah-bmm27 via Commons Wikimedia.

To better address the issue, there is the boron trifluoride molecule, BF ₃ (top image).

The fluorine atom is much more electronegative than the boron atom, and therefore the BF bonds are polar. However, the BF ₃ molecule is symmetric (trigonal plane) and entails vector cancellation of the three BF moments.

Thus, apolar molecules are also generated, even with the existence of polar bonds. The generated polarity can be balanced by the existence of another polar link, of the same magnitude as the previous one, but oriented in the opposite direction; as it happens in BF ₃.

Characteristics of an apolar molecule

Symmetry

For the effects of the polar bonds to cancel each other out, the molecule must have a certain geometric structure; for example, linear, the easiest to understand at first glance.

This is the case of carbon dioxide (CO ₂), which has two polar bonds (O = C = O). This is because the two dipole moments of the C = O bonds cancel when one is oriented towards one side, and the second towards the other, at an angle of 180 °.

Therefore, one of the first characteristics to take into account when evaluating the “apolarity” of a molecule from a bird's eye view is to observe how symmetric it is.

Suppose that instead of CO ₂ we have the molecule COS (O = C = S), called carbonyl sulfide.

Now it is no longer an apolar molecule, since the electronegativity of sulfur is less than that of oxygen; and therefore, the dipole moment C = S is different from that of C = O. As a result, COS is a polar molecule (how polar is another matter).

The image below graphically summarizes everything just described:

Dipole moments of CO2 and COS molecules. Source: Gabriel Bolívar.

Note that the dipole moment of the C = S bond is less than that of the C = O bond in the COS molecule.

Electronegativity

Electronegativity on the Pauling scale has values ​​between 0.65 (for francium), and 4.0 (for fluorine). In general, halogens have a high electronegativity.

When the difference of the electronegativity of the elements that form a covalent bond is less than or equal to 0.4, it is said to be nonpolar or nonpolar. However, the only molecules that are truly apolar are those formed by bonds between identical atoms (such as hydrogen, HH).

Intermolecular forces

For a substance to dissolve in water, it must interact electrostatically with molecules; interactions that apolar molecules cannot perform.

In apolar molecules, their electrical charges are not confined to one end of the molecule, but are symmetrically (or homogeneously) distributed. Therefore, it is unable to interact through dipole-dipole forces.

Instead, apolar molecules interact with each other through London's scattering forces; These are instantaneous dipoles that polarize the electronic cloud of the atoms of neighboring molecules. Here molecular mass is a predominant factor in the physical properties of these molecules.

How to identify them?

-Perhaps one of the best methods to identify an apolar molecule is its solubility in different polar solvents, being generally not very soluble in them.

-In general, apolar molecules are gaseous in nature. They can also be to form immiscible liquids with water.

-Apolar solids are characterized by being soft.

-The forces of dispersion that hold them together are generally weak. Because of this, their melting or boiling points tend to be lower than those of compounds of a polar nature.

-Apolar molecules, especially in liquid form, are poor conductors of electricity, since they lack a net electrical charge.

Examples

Noble gases

Although they are not molecules, noble gases are considered apolar. If it is assumed that two of its atoms, He-He, interact for short periods of time, said interaction could be considered (half) as a molecule; molecule that would be apolar in nature.

Diatomic molecules

Diatomic molecules, such as H ₂, Br ₂, I ₂, Cl ₂, O ₂, and F ₂, are apolar. These have the general formula A ₂, AA.

Hydrocarbons

What if A were a group of atoms? It would be before other apolar compounds; for example, ethane, CH ₃ -CH ₃, whose carbon skeleton is linear, CC.

Methane, CH ₄, and ethane, C ₂ H ₆, are apolar molecules. Carbon has an electronegativity of 2.55; while the electronegativity of hydrogen is 2.2. Therefore, there is a low intensity dipole vector, oriented from hydrogen towards carbon.

But, due to the geometric symmetry of the methane and ethane molecules, the sum of the dipole vectors or dipole moments in their molecules is zero, so there is no net charge on the molecules.

In general, the same happens with all hydrocarbons, and even when there are unsaturations in them (double and triple bonds), they are considered nonpolar or low polarity compounds. Also, cyclic hydrocarbons are apolar molecules, such as cyclohexane or cyclobutane.

Others

The molecules of carbon dioxide (CO ₂) and carbon disulfide (CS ₂) are apolar molecules, both with a linear geometry.

In carbon disulfide, the electronegativity of carbon is 2.55, while the electronegativity of sulfur is 2.58; so both elements have practically the same electronegativity. There is no generation of a dipole vector and therefore the net charge is zero.

Likewise, there are the following molecules CCl ₄ and AlBr ₃, both apolar:

CCl4 and AlBr3 molecules. Source: Gabriel Bolívar.

In aluminum tribromide, AlBr ₃ happens the same as with BF ₃, at the beginning of the article. Meanwhile, for carbon tetrachloride, CCl ₄, the geometry is tetrahedral and symmetric, since all the C-Cl bonds are the same.

Similarly, molecules with the general formula CX ₄ (CF ₄, CI ₄ and CBr ₄), are also nonpolar.

And finally, an apolar molecule can even have octahedral geometry, as is the case with sulfur hexafluoride, SF ₆. In fact, it can have any geometry or structure, as long as it is symmetrical and its electronic distribution is homogeneous.

References

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